Motion control or movement limitation is important in kinetic assembly's, especially related to precise movement control such as allowing movement in one axis and significantly restricting movement in other axes. In one application, for a Stirling cryocooler as an example, a positive displacement piston type compressor is utilized and although this type of compressor is well known in its basic form, there are desired modifications to the piston type compressor for increasing reliability, reducing maintenance, and having better system operational characteristics. One particular focus would be upon the mechanical aspects of the piston compressor being in particular the bearings and seals, that are the necessary evils of creating the kinematic hardware to cause reciprocating motion upon a piston, wherein a shaft typically rotates in supports called bearings and the shaft has an offset crank that connects to a pivotally connected rod that moves the piston in a reciprocating manner having a contacting slip fit within a cylinder. Thus, the two basic additional items needed are the bearings that support the shaft and a fluid seal as between the piston and the cylinder; both the bearing and the seal operate dynamically, meaning they must work at the interface of relatively moving surfaces thus causing inevitable wear-equating to having replacement/maintenance issues. In addition, the bearings and seals have to have further support systems for their continuous use, as the bearings need lubrication-requiring another system to provide the bearing lubrication, further the seal also needs lubrication-sometimes it can be sacrificially included in the seal material, or the seal can fly on a fluid film based upon clearances and various configurations, or the lubrication from the bearing system can be used. In the interest of value engineering, elimination of parts and ancillary systems is ideal, while of course keeping the original function, thus in the present case elimination of the bearings and seals would be a significant step in improving the piston compressor, plus this would greatly expand the applications available to the piston compressor involving very high or low operating temperatures, or for example caustic or corrosive environments, or applications where regular maintenance access would be very difficult.
The key engineering concept would be to create the same function, being the reciprocating movement of the piston within the cylinder without the bearings or seals, by substituting other structure in their place that would not need lubrication, thus not having periodic wear, maintenance, and replacement needs. An approach is to eliminate dynamic surface interfaces, as it is these interfaces that need lubrication of some type and have wear, plus these interfaces increase the risk of failure via overheating or having surface to surface contact that can cause seizing, welding, and freezing together of the components at the dynamic interface, or excessive seal leakage or blowby of fluid in-between the piston and cylinder. Often it is difficult to reliably predict the threat of imminent dynamic surface interface failure due to the difficulty of getting instrumentation into the dynamic surface interface that could possibly warn of an impending failure of seizing, welding, and freezing together of the components at the dynamic interface, or excessive seal leakage or blowby of fluid in-between the piston and cylinder, so this often undesirably results in sudden and unexpected failure of the piston compressor. Thus, elimination of the contacting dynamic interface would be greatly desired, which would also eliminate the complete dependence on the critical dynamic fluid film that exists between the shaft and the bearing, and the seal and the cylinder, because it is this fluid film that can momentarily weaken or disappear unexpectedly causing the sudden seizing, welding, and freezing together of the components at the dynamic interface, or excessive seal leakage or blowby of fluid in-between the piston and cylinder.
One solution is to go to a flexure beam structure that can accommodate creating the reciprocating movement of the piston within the cylinder, thus eliminating the shaft, bearings, offset crank, and pivotally connected rod. However, there is still the issue of the seal that resides in the dynamic interface of the piston and cylinder, which through creative flexure beam design can add an extreme amount of rigidity in a lateral axis, i.e. perpendicular to the reciprocating movement such that a close but non-contacting clearance can be achieved radially or laterally as between the piston and the cylinder to eliminate the dynamic contacting seal, thus all but eliminating all dynamically contacting interfaces within the piston compressor. Thus would virtually do away with the wearing dynamic interfaces requiring some type of fluid lubricating interfaces, greatly simplifying the mechanical structural needs of the piston compressor, and further for the most part completely eliminating maintenance, the need for auxiliary support systems (such as lubrication), and significantly reduce the opportunity of a sudden unexpected failure in the form of seizing, welding, and freezing together of the components at the dynamic interface or excessive seal leakage or blowby of fluid in-between the piston and cylinder.
In looking at the prior art in this area, in U.S. Pat. No. 5,522,214 to Beckett, et al., disclosed a flat spiral spring flexure bearing support with particular application to Stirling machines, with the flat spiral spring flexure shown in a top or plan view in FIG. 2, wherein the outer periphery is fixed and the central aperture facilitates reciprocal movement through movement of the spiral cuts in the flexible flat element. In Beckett, the use of flexures in the form of flat spiral springs cut from sheet metal materials provides support for a coaxial nonrotating linear reciprocating single piston member in power conversion machinery, such as a Stirling cycle engine or a heat pump. Beckett permits operation of the reciprocating piston member within the cylinder with little or no rubbing contact or other wear mechanisms due to a claimed high radial stiffness of the flat spiral spring flexures.
For Beckett, the relatively movable members include one member having a hollow interior structure within which the flat spiral spring flexures are located, in a stacked fashion, see FIG. 3, wherein the flat spiral spring flexures permit limited axial movement between the interconnected members or piston and prevent adverse rotational movement and radial displacement from the desired coaxial positions of the piston relative to the cylinder. Beckett requires multiple “stacking” of the flat spiral spring flexures as shown in FIG. 3, to achieve high radial stiffness and high anti-rotation rigidity, however sacrificing axial piston movement flexibility by essentially combining multiple springs in parallel wherein the spring rate “K” factor increases. Other issues with Beckett would include stress risers at the spiral cuts themselves which would be subject to fatigue failure problems and bearing metal to metal wear as between the flat sides of the adjacent flat spiral spring flexures, further on the outer and inner fixed peripheral attachment annulus area of the flat spiral spring stress concentrations can occur due to a high number of repetitive deflective stress cycles at the attachment annulus area. Also, Beckett, only provides for a single piston within a cylinder which can lead to imbalances and vibration in the entire assembly.
Continuing in the prior art, in U.S. Pat. No. 5,351,490 to Ohishi et al., disclosed is a piston displacer support means for a cryogenic refrigerator utilizing a plurality of flat piston suspension springs include a plurality of spiral slits to provide a plurality of spiral arms, see FIG. 3, that are deflectable as the piston is reciprocated within the compressor cylinder, see FIG. 1, being somewhat similar to Beckett. Ohishi also has a plurality of annular inner retainers that are secured to the piston and are adapted to sandwich the inner peripheral edges of the piston suspension springs. Further in Ohishi a plurality of annular outer retainers are secured to the compressor housing and include a plurality of projections extending inwardly from the outer ends of the spiral slits to sandwich the outer peripheral edges of the flat piston suspension springs. The novelty in Ohishi is in the stress reduction attachment annulus area at both the outer periphery and the inner periphery of the flat spiral slit plates, thus confirming the problem of flat spiral spring stress concentrations that occur due to a high number of repetitive deflective stress cycles at the attachment annulus area, potentially causing cracking and shearing failure of one of the flat spiral spring segments.
Ohishi has projections that reduce the local stress intensity upon the flat spiral spring segment and both the inner and outer periphery, the projections somewhat resemble an electrical cord having progressively thicker cord support when approaching the receptacle-thus a gradual distribution of stress that the cord/receptacle area would see, further a series of apertures are disposed in-between the inner and outer peripheries positioned at the junction of the spiral slits to eliminate sharp section transitions to also reduce stress concentration in the flat spiral spring segment. Other issues with Ohishi would include stress risers at the spiral cuts themselves which would be subject to fatigue failure problems, although Ohishi has addressed this issue with the apertures at the spiral cut junctions, however, there can still be problems along the body of the spiral cut itself for stress riser from fatigue, it the cuts are not smooth and even and also the bearing metal to metal wear as between the flat sides of the adjacent flat spiral spring flexures, although Ohishi has also taken steps to accommodate the outer and inner fixed peripheral attachment annulus area of the flat spiral spring stress concentrations via the use of the projections thus offsetting these stress concentrations that can occur due to a high number of repetitive deflective stress cycles at the attachment annulus area.
What is needed is a bearing/support flexure beam structure that can accommodate creating the reciprocating movement of the piston within the cylinder, thus eliminating the shaft, bearings, lubrication system, offset crank, and pivotally connected rod. Further in addition, the flexure beam structure needs to provide highly rigid rotational and lateral support of the piston reciprocating within the cylinder, to allow the piston to operate within the cylinder without the need for any surface contact for support, thus eliminating dynamic contacting interfaces resulting in the elimination of seals and bearings. Due to the previously identified problems with the prior art use of the flat spiral spring configuration, it would be highly desirable to not utilize this configuration, and instead use a flexure beam that eliminates the spiral cut stress problem, the spiral cut sharp corner junction stress problem and the inner and outer peripheries attachment stress riser problems of the flat spiral spring. Using a plurality of flexure beams with relatively large and consistently sized cross sections will eliminate highly cyclic fatigue induced stress risers, thus increasing the reliability and operational life of the flexure beam thus overcoming the previous problems with the prior art flat spiral spring arrangement identified in both Beckett and Ohishi.